The present invention is generally directed to toners and processes for the preparation of toner resin and toner thereof. More specifically, the present invention relates to melt mixing processes, batch or continuous, but preferably continuous processes such as, for example, extrusion for preparing crosslinked toner resins and toners thereof. Specifically, the present invention in embodiments is directed to a two step melt mixing process in which (1) a reactive base resin is melt mixed with a chemical initiator to form a highly crosslinked precursor resin, and (2) the resulting highly crosslinked precursor resin is directed, especially fed to an extruder together with additional base resin, and optionally toner pigments and/or other known toner additives. In embodiments the process of the present invention enables the dilution of a highly crosslinked precursor resin to form toner resins and toner compositions thereof. The present invention relates to processes for the preparation of partially crosslinked toner resins or heat fixable toners with, for example, excellent low temperature fixing characteristics and superior gloss and offset properties in a hot roll fixing system, and with excellent vinyl offset properties and wherein in embodiments the fuser roll life can be increased.
The toner resin can be prepared as illustrated in U.S. Pat. No. 5,227,460 and U.S. Pat. No. 5,376,494, the disclosures of which are totally incorporated herein by reference. For example, the crosslinked resin selected can be prepared as illustrated in column 13, beginning at line 27, of the 5,227,460 patent, and wherein a base resin and initiator are fed to an extruder; the base resin is melted; the molten resin and initiator are mixed; crosslinking is initiated by raising the melt temperature of the base resin and controlling the temperature along the extruder channel; retaining the polymer melt in the extruder for a sufficient residence time at a selected temperature to enable the desired amount of crosslinking; and providing high shear during crosslinking. Also, examples of base resins that can be selected for the processes of the present invention are illustrated in the '460 patent and the aforementioned copending application.
A need exists for a process to prepare toners which melt at lower temperatures than a number of toners now used with certain copying and printing machines. Temperatures of approximately 160.degree. to 200.degree. C. are often selected to fix a toner to a support medium such as a sheet of paper or transparency to create a developed image. These high temperatures may reduce or minimize the life of certain fuser rolls such as those comprised of silicone rubbers or fluoroelastomers like VITON, may limit fixing speeds; and/or may necessitate larger amounts of power to be consumed during operation of a copier or printer such as a xerographic copier which employs a method of fixing such as, for example, hot roll fixing.
Toner utilized in the electrographic process is generally prepared by mixing and dispersing a colorant and a charge enhancing additive into a thermoplastic binder resin, followed by micropulverization. As the thermoplastic binder resin, several polymers are known including polystyrenes, styreneoacrylic resins, styrene-methacrylic resins, polyesters, epoxy resins, acrylics, urethanes and copolymers thereof. As the colorant, carbon black or color pigment, such as cyan, can be selected, and as the charge enhancing additive, alkyl pyridinium halides, distearyl dimethyl ammonium methyl sulfate, and the like are known.
To fix the toner to a support medium, such as a sheet of paper or transparency, hot roll fixing is commonly used. In this method, the support medium carrying a toner image is transported between a heated fuser roll and a pressure roll, with the image face contacting the fuser roll. Upon contact with the heated fuser roll, the toner melts and adheres to the support medium, forming a fixed image. This fixing system is very advantageous in heat transfer efficiency and is especially suited for high speed electrophotographic processes.
Fixing performance of the toner can be characterized as a function of temperature. The lowest temperature at which the toner adheres to the support medium is referred to as the Cold Offset Temperature (COT), and the maximum temperature at which the toner does not adhere to the fuser roll is referred to as the Hot Offset Temperature (HOT). When the fuser temperature exceeds HOT, some of the molten toner adheres to the fuser roll during fixing and is transferred to subsequent substrates containing developed images, resulting for example in blurred images. This undesirable phenomenon is known as offsetting. Between the COT and HOT of the toner is the Minimum Fix Temperature (MFT) which is the minimum temperature at which acceptable adhesion of the toner to the support medium occurs, as determined by, for example, a creasing test. The difference between MFT and HOT is referred to as the Fusing Latitude.
Gloss performance of toner can be characterized as a function of fusing temperature. The fusing temperature at which the image attains a gloss level of 50 gloss units is referred to as the Gloss 50 Temperature, T(G.sub.50); hereinafter, unless otherwise indicated, all gloss units refer to TAPPI T480 75.degree. specular gloss. The difference between T(G.sub.50) and HOT is referred to as the Gloss Latitude. The maximum gloss level of the image in the temperature range between MFT and HOT is referred to as the Peak Gloss.
Many prior art toner resins developed have the required melt viscosity to produce images with high gloss or low gloss on plain paper, for example from about 25 to about 60 gloss units for high gloss (high gloss toner resin) and from about 1 to about 15 gloss units for low gloss (low gloss toner resin). Toners which generate high gloss images are often selected for process and highlight color applications and transparencies; toners with low gloss are generally used for matte applications. Although these properties are desired, the fixing or fusing temperature of the toners are high and usually more than 160.degree. C. This may result in high power consumption, low fixing speeds, and reduced life of the fuser roll and fuser roll bearings. Offsetting can also be a problem. Furthermore, toners containing vinyl type binder resins such as styrene-acrylic resins may have an additional problem which is known as vinyl offset. Vinyl offset occurs when a sheet of paper or transparency with a fixed toner image comes in contact for a period of time with a polyvinyl chloride (PVC) surface containing a plasticizer used in making the vinyl material flexible such as, for example, in vinyl binder covers, and the fixed image adheres to the PVC surface. Also, a number of toner resins with lower melt temperatures possess a narrow fusing latitude and have poor mechanical properties, creating too many fines during jetting which have to be removed by classification and reused. This results in increased toner cost. Furthermore, many prior art toner resins are prepared for specific uses and, therefore, there is a resin for a low gloss and another different resin for high gloss. This results in the need for a number of resin manufacturing processes which further increases the cost of the toners. These and other disadvantages are avoided or minimized in embodiments of the present invention.
There is a need for processes which can be used to prepare toner resins or toners for different applications such as high gloss or low gloss, with low fusing toner temperature below 200.degree. C., preferably below 160.degree. C., such as about 110.degree. C. to about 155.degree. C., (referred to as low fix temperature toner resin or toner, or low melt toner resin or toner), excellent offset performance, and superior vinyl offset properties. Toners which operate at lower temperatures would reduce the power needed for operation and increase the life of the fuser roll and the high temperature fuser roll bearings. Additionally, low melt toners would reduce the volatilization of release oil such as silicon oil which may occur during high temperature operation and which can cause problems when the volatilized oil condenses in other areas of the machine. In particular, low melt toners with a wide fusing and excellent gloss latitude and with acceptable toner particle elasticity are needed. Further, toners with wide fusing and excellent gloss latitude can provide flexibility in the amount of oil needed as a release agent; can minimize copy quality deterioration related to the toner offsetting to the fuser roll; and can extend fuser roll life. Furthermore, there is a need for economical processes wherein different resins for high gloss or low gloss toner are generated. These and other needs are achievable with the processes of the present invention.
To lower the minimum fix temperature of the toner, in some instances the molecular weight of the binder resin may be lowered. Low molecular weight and amorphous polyester resins and epoxy resins have been used for low temperature fixing toners. For example, attempts to use polyester resins as a binder for toner are disclosed in U.S. Pat. No. 3,590,000 and U.S. Pat. No. 3,681,106. The minimum fixing temperature of polyester binder resins can be lower than that of other materials, such as styrene-acrylic and styrene-methacrylic resins. However, this may lead to a lowering of the hot offset temperature, and as a result, decreased offset resistance and shortened fuser roll life. In addition, the glass transition temperature of the resin may be decreased, which may cause the undesirable phenomenon of blocking of the toner during storage. Furthermore, toner prepared from such a resin will produce images with undesirable crease performance and narrow fusing latitude.
U.S. Pat. No. 5,057,392, discloses a low fusing temperature toner powder which employs a polyblend of a crystalline polyester and an amorphous polyester that has been crosslinked with an epoxy novolac resin in the presence of a crosslinking catalyst. The disclosed polyblend contains a mechanical mixture of the crystalline and amorphous polyester melt blended together. The crystalline polyester is required to maintain a desired low melt temperature and the amorphous polyester is required to maintain a desired high offset temperature. In the polyblend, the amorphous polyester is partially crosslinked with the epoxy novolac resin. The disclosed toner powder requires the presence of crystalline and amorphous polyesters, and upon completion of crosslinking, the crystalline polyester recrystallizes as dispersed small particles within a matrix phase of the crosslinked amorphous polyester and epoxy resin. In a disclosed process for preparing the toner particles, the crystalline polyester, amorphous polyester resin, epoxy novolac resin, crosslinking catalyst, colorant, crystallization promoter and optional charge control agent are melt blended, preferably by an extrusion process. During melt blending, the amorphous polyester is crosslinked with the epoxy novolac resin. After melt blending the mixture is annealed to recrystallize the crystalline polyester. The disclosed melt blended mixture is not useful as a toner requiring a low melt temperature until it is annealed. In addition, the glossy image prepared on paper with toner prepared from such a mixture does not possess a wide fusing latitude, it is believed.
To prevent fuser roll offsetting and to increase fuser latitude of toners, various modifications have been made to toner compositions. For example waxes, such as low molecular weight polyethylene, or polypropylene, have been added to toners to increase the release properties, as disclosed in U.S. Pat. No. 4,513,074, the disclosure of which is incorporated herein by reference. However, to prevent offset sufficiently, considerable amounts of such materials may be required in some instances, resulting in detrimental effects such as the tendency for toner agglomeration, undesirable free flow properties and destabilization of charging properties. Also, waxes tend to degrade projection efficiency of glossy color transparencies.
Modification of binder resin structure, for example by branching, or crosslinking, when using conventional polymerization reactions may also improve offset resistance. In U.S. Pat. No. 3,681,106, for example, a polyester resin was improved with respect to offset resistance by nonlinearly modifying the polymer backbone by mixing a trivalent or more polyol or polyacid with the monomer to generate branching during polycondensation. However, an increase in degree of branching may result in an elevation of the minimum fix temperature. Thus, any initial advantage of low temperature fix may be diminished.
U.S. Pat. No. 4,797,339 discloses a modified toner resin containing a particle-to-particle ionically crosslinked resin complex. The disclosed crosslinked resin complex is obtained by reacting a cationic resin emulsion and an anionic resin emulsion. The resulting resin ion complex has a glass transition temperature of -90.degree. to 100.degree. C. and a degree of gellation of from 0.5 to 50 percent by weight, preferably 10 to 30 percent by weight. It is stated that if the degree of gellation is too high beyond 50 percent by weight, the fixability of the toner at low temperatures tends to be reduced undesirably. If it is too low below 0.5 percent by weight, scattering of the toner tends to increase undesirably. The emulsion polymerization process disclosed results in production of a sol component in the polymer (i.e., crosslinked portions which are not densely crosslinked).
A method of improving offset resistance of low melt toner is to utilize crosslinked resin in the binder resin. For example, U.S. Pat. No. 3,681,106 discloses a toner in which a crosslinked polyester, prepared using conventional crosslinking methods, is used as the binder resin. Similar disclosures for polyester resins are provided in U.S. Pat. Nos. 4,933,252 and 4,804,622.
While significant improvements can be obtained in offset resistance and entanglement resistance in toner resins, a major drawback may ensue in that with crosslinked resins prepared by conventional polymerization (that is, crosslinking during polymerization using monomer and a crosslinking agent), there exist three types of polymer configurations: a linear and soluble portion referred to as the linear portion; a portion comprising highly crosslinked gel particles which is not soluble in substantially any solvent, e.g., tetrahydrofuran, toluene and the like, and is the gel, and a crosslinked portion which is low in crosslinking density and therefore is soluble in some solvents, e.g., tetrahydrofuran, toluene and the like, and is the sol. Also, there are monomeric units between the crosslinked polymer chains. The presence of highly crosslinked gel in the binder resin increases the hot offset temperature, but at the same time the low crosslink density portion or sol increases the minimum fix temperature. An increase in the amount of crosslinking in these types of resins results in an increase not only of the gel content, but also of the amount of sol or soluble crosslinked polymer with low degree of crosslinking in the mixture. This results in an elevation of the minimum fix temperature, and as a consequence, in a reduction or reduced increase of the fusing latitude. In addition, a drawback of embodiments of crosslinked polymers prepared by conventional polycondensation in a reactor with low shear mixing, for example, less than about 0.1 kW-hr/kg, is that as the degree of crosslinking increases, the gel particles or very highly crosslinked insoluble polymer with high molecular weight grow larger. The large gel particles can be more difficult to disperse pigment in, causing the formation of unpigmented toner particles during pulverization, and toner developability may thus be hindered. Also, compatibility with other binder resins may be relatively poor and toners containing vinyl polymers often show vinyl offset.
U.S. Pat. No. 4,533,614 discloses a loosened crosslinked polyester binder resin which provides low temperature fix and good offset resistance, and wherein metal compounds were used as crosslinking agents. Similar disclosures are presented in U.S. Pat. No. 3,681,106 and Japanese Laid-Open Patent Applications 94362/1981, 116041/1981 and 166651/1980. As indicated in the '614 patent, incorporation of metal complexes, however, can influence unfavorably the charging properties of the toner. Also, with color toners other than black (e.g., cyan), metal complexes can adversely affect the color of pigments. It is also known that metal containing toners can have disposal problems in some geographical areas, such as for example in the State of California, U.S.A. Metal complexes are often also costly.
Many processes are known for effecting polymerization reactions, including reactive extrusion processes, for both initial polymerization reactions employing monomers or prepolymers, and for polymer modification reactions, such as graft, coupling, crosslinking and degradation reactions. U.S. Pat. No. 4,894,308 and U.S. Pat. No. 4,973,439, for example, disclose extrusion processes for preparing electrophotographic toner compositions in which pigment and charge control additive were dispersed into the binder resin in the extruder. However, in each of these patents, there is no suggestion of a chemical reaction occurring during extrusion.
An injection molding process for producing crosslinked synthetic resin molded articles is disclosed in U.S. Pat. No. 3,876,736 in which polyolefin or polyvinyl chloride resin and crosslinking agent were mixed in an extruder, and then introduced into an externally heated reaction chamber outside the extruder wherein the crosslinking reaction occurred at increased temperature and pressure, and at low or zero shear.
In U.S. Pat. No. 4,089,917, an injection molding and crosslinking process is disclosed in which polyethylene resin and crosslinking agent were mixed in an extruder and reacted in reaction chambers at elevated temperature and pressure. Heating of the resin mixture occurred partially by high shear in inlet flow orifices. However, the crosslinking reaction in this process occurs in the reaction chambers at low or zero shear, and the final product is a thermoset molded part, and thus is not considered useful for toner resins.
A process for dispensing premixed reactive precursor polymer mixtures through a die for the purposes of reaction injection molding or coating is described in U.S. Pat. No. 4,990,293 in which polyurethane precursor systems were crosslinked in the die and not in the extruder. The dimensions of the die channel were determined such that the value of the wall shear stress was greater than a critical value in order to prevent gel buildup and consequent plugging of the die. The final product is a thermoset molded part, not considered useful as a toner resin.
The processes disclosed in U.S. Pat. Nos. 3,876,736; 4,089,917 and 4,990,293 are not considered reactive extrusion processes, primarily because the crosslinking occurs in a die or a mold, and not in an extruder, and the crosslinking takes place at low or zero shear. These processes are for producing engineering plastics such as thermoset materials which cannot be remelted once molded, and thus are not useful in toner applications.
In U.S. Pat. No. 5,395,723, the disclosure of which is incorporated herein by reference, a polyester low melt toner resin is described which is prepared by reactive extrusion and which is suitable for low gloss matte application such as for example matte black images. Also, in copending application U.S. Ser. No. 334,012, filed concurrently herewith, there is disclosed a polyester toner resin which is prepared by reactive extrusion and which is suitable for high gloss or process color application and which has low fix temperature, excellent offset resistance, wide fusing latitude and possesses minimized or substantially no vinyl offset. Also, in U.S. Pat. No. 5,227,460 there is disclosed low melt toners with reactive extruded resins and wherein the microgel particles can be present in an amount of from about 0.001 percent to about 50 percent, and other amounts, see column 7, beginning at line 23. The disclosures of each of the aforementioned documents are totally incorporated herein by reference.
There is a need for one process for the preparation of low gloss or high gloss, low melt toner resin or toner with excellent offset resistance, wide fusing and excellent gloss latitude, and which toner possesses minimized or substantially no vinyl offset, and wherein the toners can be selected for the generation of matte or glossy applications and transparencies.